The present invention relates to a reader device (hereinafter termed "laser scanner" for the sake of brevity) for reading characters with different reflectance values, such as for example bar codes.
The use is known of two or more lasers, suitable to be focused at different distances, inside a single scanner, so as to increase the depth of field for the reading distance in bar code readers. An example of this technique is described in European Patent Application published on Apr. 15, 1992 under Serial No. 0480348.
In the known system, the lasers related to the various distances are activated sequentially and alternately, one for each scanning operation, and if one scan is found to contain the code, all subsequent scans are performed by the same laser, until the code remains present; once this situation ends, scanning resumes with cyclic alternation of the lasers.
Another known method entails the activation of the laser related to the reading distance by measuring, by means of a barrier of photocells external to the scanner, the height of the object that bears the code to be read.
These systems have drawbacks: in the first case, the actual scan rate is equal to the real scan rate divided by the number of lasers being scanned; the second method requires the use of external photocells and is furthermore effective only if the objects on which the code is placed have a regular shape. A defect of both systems is that only one focusing is possible during an individual scan. In other words, it is not possible to simultaneously read two codes that are aligned but located on planes that are at different distances from the scanner.